1. Field of the Invention
The present invention relates to a non-impact recording method and especially to a liquid jet recording method suitable for digital copying machine, fascimile and printer.
More particularly, the present invention relates to a liquid jet recording method of the type in which the recording liquid is jetted from an orifice to form flying droplets by use of thermal energy and the droplets are deposited on the surface of a recording material.
2. Description of the Prior Art
The non-impact recording method is a very attractive recording method for its particular advantage that the noise generated during recording is negligibly small. Among various non-impact recording methods hitherto known, the liquid jet recording method (ink jet recording method) is the one having the most potential. It makes possible recording at higher speed and on any plain papers without need for any particular processing for fixing. Owing to these advantages, many attempts have been made to develop apparatus for carrying out the liquid jet recording method, some of which have already been put to practical use and some of which are now under improvement for practical use.
Among others, the liquid jet recording method as disclosed in German Laid-Open Specification (DOLS) No. 2,843,064 is attracting attention in the art. The recording method disclosed there is distinguished from other similar liquid jet recording methods in the feature that the motive force for jetting liquid droplets is obtained by applying to the liquid the thermal energy serving as droplet forming energy.
According to the recording method disclosed in the patent publication, the liquid is subjected to the action of thermal energy to cause a change of state (such as generation of air bubbles) accompanied by an abrupt increase of volume. From this change of state there is produced an acting force by which the liquid is jetted from the orifice as liquid droplets flying toward a recording material on which the droplets stick to produce a record.
An advantage of the recording method disclosed in DOLS No. 2,843,064 is that it is applicable with particular effectiveness to the so-called drop-on-demand recording process. Another advantage thereof is found in the fact that it enables one to realize a multi-orifice recording head having a plural number of orifices very closely arranged over the full line width of the recording head part. With such a multi-orifice recording head, images of high resolution and high quality can be obtained at high speed.
Although the liquid jet recording method described above has many advantages over other recording methods, it involves a problem relating to the durability of the recording head. When images of high resolution and high quality are to be recorded for a long time at higher speed than that allowable at present or when it is wished to prolong the useful life of the recording apparatus to a great extent, a further improvement of the recording head is required regarding the repeating useful life (durability) of the head.
The durability of the recording head used for carrying out the above recording method is determined by various factors. One of the factors is, of course, the life of the electro-thermal transducer used therein. Another factor is the deposition of solid matter on the surface of the transducer.
A typical structure of the recording head used for carrying out the above recording method comprises orifices provided from which to jet liquid, a liquid jet part in communication with said orifices and an electro-thermal transducer. The liquid jet part has a heat action portion where thermal energy acts on the liquid to form flying liquid droplets. The electro-thermal transducer serves as means for generating the thermal energy acting on the liquid when said heat action portion is filled. The electro-thermal transducer is so disposed as to constitute a portion of the liquid flow channel and also to be in contact with the liquid introduced through an inlet at the heat action portion through a heat action surface. The heat action portion is a portion where the droplet forming energy acts. The heat action surface through which the electro-thermal transducer is in contact with the introduced liquid is a surface through which the energy acts on the liquid.
The electro-thermal transucer includes a heat generating part comprising a heating resistor layer and a pair of electrodes for applying an electric signal to the heating resistor layer.
The purpose for which the above structure of the recording head has been employed is to make the generated thermal energy as droplet forming energy most effectively and efficiently act on the recording liquid present at the heat action area.
When the composition of the recording liquid requires it, for example, when water is used as the liquid medium for recording liquid, there may be provided on the heating resistor layer at the heat generation part an upper layer for preventing electric leak between the pair of electrodes through the recording liquid as well as for protecting the heating resistor layer against the action of the recording liquid or against thermal oxidation.
With the above described recording head the liquid droplets are formed according to the principle of the liquid jet recording method previously described, which is as follows:
When an electric signal is applied to the electro-thermal transducer, there is produced, as droplet forming energy, an amount of thermal energy which acts on the recording liquid present at the heat action area. By this action there is caused a state change of the recording liquid accompanied with an abrupt increase of its volume. The recording liquid at the heat action area reaches the vaporizable state on a very short time in the order of .mu.sec. As a result, bubbles are formed and grow rapidly in a moment. Thereby the recording liquid existing in the liquid channel between the heat action portion and the orifice is jetted from the orifice in the form of flying liquid droplets.
During the above process of repeated formation and extinction of bubbles, the recording liquid is subjected to the action of high temperature heat which is apt to cause a chemical change of the recording liquid, particularly when there is used a thermally unstable recording liquid. This chemical change of the recording liquid often leads to formation and deposition of insoluble matter in the heat action area. In the worst case, the recording head becomes unable to jet liquid any more. To perform recording for a long time at high speed employing the above recording apparatus, it is essential to set the optimum operational conditions for the recording head while improving the stability of the recording liquid.
As another problem of the above described liquid jet recording method using thermal energy it has also been found that during recording at high speed or during recording for a long time the performance of liquid jet from the recording head sometimes suffers in respect of jet responsiveness, jet efficiency, jet stability, etc. This problem is attributable to undesirable bubble generation in the recording liquid present within the liquid flow channel. During the use of the recording head, there are often generated within the channels undesirable bubbles which may obstruct the flow of recording liquid in the area near the small jet orifice. Also, such undesirable bubbles absorb a part of the liquid jet motive force generated by jetting energy generating means such as a heating resistor. As a consequence of it, the responsiveness of the recording liquid to the signal is reduced. The recording head can no longer jet liquid in a stable manner in response to the applied signal.
Such unfavourable bubble generation is attributable, in substance, to the fact that the motive force for jetting liquid droplets is obtained from the change of state of the recording liquid (especially from the generation of bubbles therein by the action of thermal energy). In view of this fact, it is natural that undesirable bubbles are very easily generated in the process of recording and that the performance of the apparatus including jet responsiveness, jet efficiency and jet stability is easily and greatly affected by such undesirable bubbles.
Once generated, such undesirable bubbles within the heat action area can not disappear in a short time. Rather, the generation of such undesirable bubbles may be accelerated by dissolved gas in the recording liquid.
To solve the problem of generating undesirable bubbles in the recording liquid there have been proposed and used various methods. For example, it is known to use an air-tightly formed liquid reservoir thereby reducing the amount of dissolved gas therein. It is also known to add an oxygen absorber to the recording liquid for purpose of reduction of the dissolved gas. Another known method is to provide a particular bubble exhaust path above the heat action portion of the recording head. The exhaust path is in communication with the heat action area so that the undesirable bubbles flow into the path owing to their own buoyancy and the bubbles can be trapped in the path above the heat action area.
However, it is difficult to obtain satisfactory results from the above known methods. For example, even when the liquid reservoir is made of any air-tight material, gas (air) may penetrate into the reservoir through the material and the amount of dissolved gas in the recording liquid within the reservoir may reach the saturation after a certain time period. The addition of oxygen absorber to the recording liquid may have an adverse effect on the property of the recording liquid.
In the case a recording head used to jet liquid droplets by means of thermal energy, it is desirable that a rapid change of state of the recording liquid be caused for purpose of improvement in responsiveness and efficiency of liquid jet. To this end, sometimes gas is intentionally dissolved in the recording liquid to positively use the dissolved gas for the desired rapid change of state. Therefore, the method useful for reducing the amount of dissolved gas is not always the best method for maintaining the good performance of the recording head.
The provision of a particular bubble trap path above the heat action area of the recording head also has a difficulty in smoothly removing the undesirable bubbles. In this case, the upward movement of the bubbles into the trap path relies upon buoyancy of the bubble only. Since the trap path is very narrow, the bubbles can not be always smoothly removed through the path.